The present invention relates to adaptive arrays, and more particularly to an adaptive array configured to provide strong cancellation of interference over a broad bandwidth.
In an adaptive array, it is often necessary to compensate for multipath, element-to-element time delay, channel mismatch, and other sources of channel-to-channel distortion. This compensation is required to achieve strong cancellation of interference.
Adaptive array processing is employed to process the signals from an array of sensors to attenuate or remove unwanted signals (i.e., interference) under changing conditions. An example of a particular adaptive array application is the coherent sidelobe canceller, comprising a main directional antenna and at least one omnidirectional antenna. The main channel receives the output from the main antenna, which comprises the desired signal plus the unwanted noise (such as a jammer signal which may be received as sidelobe energy). The output of the omnidirectional antenna comprises the unwanted noise (as well as some of the wanted signal) and is fed to a second channel for adaptive filtering to adjust the amplitude and time delay of the unwanted noise so that, upon combination of the main and second channel outputs the unwanted noise is effectively cancelled. If the incident energy is narrowband, the time delay adjustment performed by the adaptive filter may simply be a phase change, since the time delay of the signals at the respective antennas resulting from the same source may be accurately characterized as a phase delay at a particular frequency. When the signals are broadband, however, there is no single phase shift accurately characterizing the time delay.
One approach to providing broadband compensation is suggested by the paper "An Alogrithm for Linearly Constrained Adaptive Array Processing," by Otis L. Frost, Proceedings of the IEEE, Vol. 60, No. 8, August, 1972, pages 926, et seq. This approach utilizes tapped delay lines to achieve the desired noise cancellation, and is therefore relatively expensive to implement. Other methods previously proposed to accomplish broadband compensation by bandwidth partitioning use either channelized filters or the Discrete Fourier Transform (requiring digitization of the data) to divide the bandwidth into smaller sub-bands.
It would therefore represent an advance in the art to provide a system which avoids the need to digitize the input signals, can accommodate broader bandwidths, and is compatible with analog adaptive weighting circuits.